The present invention relates to voltage reference circuits and devices, and more particularly to a radiation-hardened temperature-compensated voltage reference device comprising a pair of series-connected diodes, including a method of matching the temperature coefficients of voltage of the diodes, and controlling the neutron coefficients of voltage of the diodes.
Many electronic circuits require one or more voltage reference sources that provide a constant, stable, voltage value for use within the circuit as a reference voltage from which the operation of the circuit, or portions of the circuit, can be controlled. Often, the voltage reference source must be a precision reference source that provides the desired voltage value regardless of changes in the environment to which the voltage reference source is exposed. The most common change in environment that can affect a voltage reference source is temperature. However, changes in neutron fluence (radiation levels to which the circuit is exposed) can also have an adverse impact on the voltage value provided. Those skilled in the art have long sought for a precision voltage reference source that is immune to changes in both temperature and neutron fluence.
The most widely used approach to temperature compensated precision voltage references is a reverse biased avalanche diode in series with a forward biased P-N junction diode. The voltage arising across this series combination, when a specified current is allowed to flow therethrough, is quite insensitive to temperature variations. This configuration achieves temperature compensation by offsetting the positive temperature coefficient of voltage of the avalanche diode with the negative temperature coefficient of voltage of the forward biased diode. By this means, temperature coefficients as low as a few parts per million (ppm) can be achieved. Unfortunately, however, exposure to the adverse effects of neutron irradiation significantly reduces the performance of such circuits at neutron fluence levels which are unacceptably low. Thus, even though a reverse-biased voltage reference diode in series with a forward-biased p-n diode may provide a temperature compensated voltage reference source, such source is not necessarily stable when exposed to radiation.
Voltage reference diodes typically comprise a p-n junction that is operated in the reverse direction at sufficient bias to cause either avalanche or zener breakdown. Advantageously, the desired property of a voltage reference diode is that very little current flows therethrough until a particular reverse voltage, termed the "breakdown voltage" (V.sub.BR) is reached. At the breakdown voltage, either the zener or the avalanche processes cause the effective impedance of the diode to become very small, thereby allowing a large range of currents to flow through the diode without significantly altering the voltage drop across the diode. The effect of this very small impedance is to maintain the voltage drop across the reference diode essentially constant for a large range of current values flowing through the reference diode.
Voltage reference diodes are used as a voltage reference source in circuits where a constant voltage reference is required even though there may be variations in the power supply voltage used to reverse bias the p-n junction of the diode. However, so long as the voltage applied to the diode biasing circuit remains greater than the breakdown voltage, the voltage measured across the diode (i.e., the reference voltage) remains essentially constant. For many applications, however, even small variations in the reference voltage destroys the usefulness of the reference diode.
Changes in the breakdown voltage caused by temperature variations can be offset (compensated for) by including a forward biased diode in series with the reverse-biased voltage reference diode, as has been indicated. However, the breakdown voltage of an avalanche reference diode depends inversely upon impurity doping levels. Unfortunately, these doping levels can be altered when the device is exposed to radiation (neutron fluence). In effect, neutron-induced traps decrease the effective doping levels of the p-n junction, which in turn causes the breakdown voltage to increase. Moreover, exposure to radiation causes the density of the scattering centers within the p-n junction to increase, which also disadvantageously causes the breakdown voltage to increase.
Similarly, the breakdown voltage of a zener reference diode is also adversely affected by exposure to radiation. However, in a zener diode, the breakdown voltage decreases with neutron fluence. Zener breakdown results from band to band tunneling, i.e., carriers tunneling from the conduction band of a heavily doped n-region across the forbidden gap to the valence band of a heavily doped p-region. The actual breakdown voltage exhibited by the zener diode is at least partially a function of the amount of tunneling that occurs. A large amount of tunneling results in a lower breakdown voltage than does a lesser amount of tunneling. Neutron fluence causes increased traps in the forbidden gap. These traps provide additional sites to which the carriers can tunnel. Hence, neutron fluence tends to decrease the breakdown voltage of a zener diode.
It is also known in the art to employ two diodes in series for the purpose of providing some measure of radiation hardening. (As used herein, "radiation hardening" refers to the ability of a device to withstand neutron fluence without being adversely affected.) A first diode is forward biased and a second diode, an avalanche breakdown diode, is reversed biased. With low forward current operation, the voltage of the forward biased diode decreases with neutron fluence, while the avalanche breakdown voltage of the reverse biased junction increases with neutron fluence. These effects compensate each other, and provide some measure of radiation hardening.
Unfortunately, at low to moderate neutron fluences, the lifetime degradation of the forward biased diode in such a series diode combination dominates, and the desired compensation is lost (or at least reduced below acceptable levels). The shift in reference voltage for neutron fluences in the range of 10.sup.12 n/cm.sup.2 to 10.sup.13 n/cm.sup.2 for such prior art diode combinations can vary over a wide range. While gold doping of the forward biased diode (or other means) may be used to "harden" the device to reduce the neutron-induced shift of the forward voltage drop, a voltage change of 5 millivolts or more can still be expected for neutron fluences at levels of 10.sup.14 n/cm.sup.2. For many high precision applications, a reference voltage is required that is stable to within plus or minus one millivolt (mv) or less when exposed to a high neutron fluence (&gt;10.sup.14 n/cm.sup.2). Hence, there is a need in the art for a more stable radiation-hardened temperature compensated voltage reference source.